Can Satyr Grovedancer effectively turn off my opponents monstrous (Hundred-Handed One) before they activate it? As in I put a +1/+1 counter on their monstrous creature?
Satyr Grovedancer
No.
Being monstrous is independent from having +1/+1 counters, so adding a +1/+1 counter on a creature that is not monstrous does not prevent that creature from becoming monstrous.
701.30. Monstrosity
701.30a "Monstrosity N" means "If this permanent isn't monstrous, put N +1/+1 counters on it and it becomes monstrous."
701.30b Monstrous is a designation that has no rules meaning other than to act as a marker that the monstrosity action and other spells and abilities can identify. Only permanents can be or become monstrous. Once a permanent becomes monstrous, it stays monstrous until it leaves the battlefield. Monstrous is neither an ability nor part of the permanent's copiable values.
If you are talking about "adapt" then, yes. Putting a +1/+1 counter on a creature makes the Adapt ability do nothing.
That is, the Adapt ability is activated, when it resolves, it checks if the creature has +1/+1 counters and then if there are none, it puts the +1/+1 counters, otherwise it does nothing.
It will, however, turn off adapt (and, speaking of Ravnica, it also makes unleashed creatures with no +1/+1 counters on them incapable of blocking), but not monstrosity or renown.
Private Mod Note
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Rollback Post to RevisionRollBack
Card advantage is not the same thing as card draw. Something for 2B cannot be strictly worse than something for BBB or 3BB. If you're taking out Swords to Plowshares for Plummet, you're a fool. Stop doing these things!
Moved to Rulings, Card tag added - Wildfire393
No.
Being monstrous is independent from having +1/+1 counters, so adding a +1/+1 counter on a creature that is not monstrous does not prevent that creature from becoming monstrous.
If you are talking about "adapt" then, yes. Putting a +1/+1 counter on a creature makes the Adapt ability do nothing.
That is, the Adapt ability is activated, when it resolves, it checks if the creature has +1/+1 counters and then if there are none, it puts the +1/+1 counters, otherwise it does nothing.
On phasing: